The present invention is directed to a fluidized bed reactor, and, particularly, to a fluidized bed reactor utilizing a conical-shaped support for the bed through which uncombusted material is removed from the reactor and a method for operating the fluidized bed reactor.
In fluidized bed reactors for combusting particulate material, the material to be combusted is generally fed over or into a bed of granular material, usually sand. In the past, two major types of surfaces have been used to support the bed of granular material, namely bar grates and plate grates.
A bar grate is designed to permit air or other gases to pass up through the bed and to permit refuse in the form of uncombustible tramp material and/or agglomerated material to pass through the parallel spaces formed between adjacent bars and to be removed from the bottom of the reactor. In this type of grate, air is provided to a manifold that distributes the air to individual hollow bars connected to the manifold. The bars, in turn, have air nozzles that distribute the fluidizing air into the bed. An example of such a conventional bar grate is disclosed in U.S. Pat. No. 4,075,953 to Sowards, specifically in the embodiment depicted in FIG. 11 of that patent, and in U.S. Pat. No. 3,892,046 to Cooke.
A plate grate, unlike a bar grate, does not permit tramp material and/or agglomerated material to be removed from the bottom of the reactor. Instead, the plate, which is usually in the form of a flat surface, has air nozzles that distribute the fluidizing air into the bed. Air is supplied to the nozzles from an air box located below the plate. An example of such a conventional plate grate is also disclosed in U.S. Pat. No. 4,075,953, supra, specifically in the embodiment depicted in FIG. 1 of that patent, and in U.S. Pat. No. 3,907,674 to Roberts et al.
The above-discussed prior art grates have certain disadvantages that can limit the operating times of the fluidized bed. The bar grate, despite having parallel spaces between the bars through which granular material, tramp material, and/or agglomerated material can fall, tends to restrict the flow of material or to accumulate material as it passes between the parallel sides of adjacent bars. After a period of operation, the upper surface of the grate becomes covered by a static layer of tramp material and/or agglomerated material. Moreover, granular bed material tends to enter the nozzles and accumulate in the bars, which bars are difficult to clean out. As a result, the fluidization of the bed, along with the effectiveness of the reactor, decreases. Another disadvantage of the prior art bar grates is that since the bar grate extends along the entire base of the reactor, the hopper that is located below the reactor must be coextensive with the entire reactor diameter. For large diameter reactors, the cost of the required hopper can be significant.
The plate grate suffers from the severe shortcoming of having no means by which tramp material and/or agglomerated material can be removed from the entire bed during operation. Such material can only be removed by shutting down the bed.
In addition to the bar and plate grates discussed above, conical-shaped support surfaces also have been used to support a bed of granular material. In U.S. Pat. No. 4,177,742 to Uemura et al. a conical-shaped support surface with a centrally disposed port is disclosed through which tramp material and/or agglomerated material is removed from the reactor. If the angle of inclination of the support surface is increased, the removal of tramp material and/or agglomerated material is facilitated. However, when the angle of inclination is increased, the height of the bed, particularly near the center of the reactor, will also increase. Consequently, the energy required to fluidize the bed increases. This is particularly significant with large diameter reactors.